v0.12.1: stop asking where the music is when config.json already says

Restarting on the machine that mounts the drive at /media/trav/muzak opened
"LinTunes can't find your music folder", offering ~/Music. The library's only
music_folder is the other machine's mount point (/run/media/trav/tummult/...)
because this library predates 0.10 and has no music_folder_rel yet — but
config.json has held the right path all along under music_root, which
`--music-root … --save-config` writes and the importer stores as
library.music_folder. resolve() only looked for music_folder_override, a key
that exists solely because the prompt writes it, so a library imported the
normal way could never satisfy the check that decides whether to prompt.

music_root is now a machine-local candidate alongside music_folder_override,
consulted in the same last-resort position and subject to the same must-exist
rule, so a stale one still reports "missing" rather than resolving to a phantom
tree. Read-only: nothing writes it back, and the newer key still wins when both
are present.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y9ZEFi4qNJ39FMiBtiAxy2

CLAUDE.md

@@ -140,7 +140,10 @@ persistence) → GUI (Qt widgets that read the manager and connect to its signal
   floor between versions), `music_folder_rel` relative to the data dir (the
   portable one, same trick as `Track.location`), and `music_folder_set_at`, a
   stamp so a merge prefers the newest *setting* rather than the newest *file*.
-  `resolve()` tries rel → legacy → the per-machine override in `config.json`,
+  `resolve()` tries rel → legacy → the per-machine override in `config.json`
+  (both `music_folder_override` *and* the older `music_root` that
+  `--music-root … --save-config` writes — an imported library already
+  knows where its music is on this machine, so don't ask),
   and every candidate must **exist** — the override is consulted last so a
   machine that once needed one isn't pinned to it forever. Two rules that bite:
   `LibraryManager.set_music_folder` must call `mark_library_settings_dirty()`